This invention relates to the field of optical filters and more particularly to tunable optical filters.
Optical bandpass filters whose transmission characteristics may be controlled by the application of DC voltages are useful in a variety of different applications, such as optical signal processing, high resolution spectroscopy, solar physics, astrophysics, high pressure tunable lasers, and infrared focal plane technology.
A number of such filter designs are known, including narrow band optical tunable filters known as Solc filters, Lyot-Ohman filters, and chirped multi-layer filters. An electro-optic tunable filter of the Lyot-Ohman type, for example, was first demonstrated in the visible spectrum by Billings. See B. H. Billings, 37 Journal of the Optical Society of America 738 (1947). In this filter, three quartz plates having thicknesses in the ratio 1:2:4 provide fixed phase retardations. Three ADP plates are employed as the tunable phase retardation plates.
The Lyot-Ohman type of filter, however, as well as the other types mentioned above, requires many polarizers, large crystals, and many electrodes in order to achieve a wide spectral tuning range. Because of the large number of oriented crystal layers which are required, reflection losses and surface scatterings at the crystal layer surfaces are major causes of low transmission through such prior art filters. In addition, in most tunable filter applications, the longitudinal electro-optic effect must be utilized so that a large aperture may be realized. This constraint, however, creates a serious electrode problem for both the Lyot-Ohman and Solc type of filter, since each crystal plate in such filter designs must then have associated with it a pair of electrodes through which the filtered light must pass. Such multiple electrodes tend to further decrease the transmission of incident light through the filter and thus an electro-optic tunable filter having a minimum number of electrodes is desirable.